1. Field of the Invention
The invention relates to a power amplifier for amplifying an electric signal in an operational frequency range, the power amplifier comprising switching means for generating a block wave signal by alternately switching the block wave signal to a first supply voltage or a second supply voltage, filter means for generating a power output signal by low-pass filtering the block wave signal, input means for receiving the electric signal and driving the switching means, and a control circuit coupled to the output power signal and the input means for controlling the power amplifier.
2. Description of the Related Art
A switching power amplifier is known from U.S. Pat. No. 6,297,692. This patent describes pulse modulation amplifiers, also called pulse-width modulation amplifiers or class D amplifiers. In such amplifiers, there is an input stage for receiving an electrical input signal to be amplified, a modulating/switching stage and a filter. The modulating/switching stage generates a block wave signal having a frequency that is much higher than the highest frequency in the operational frequency range of the electric signal to be amplified. The pulse-width ratio of the block wave is modulated so that the average value of the block wave signal is proportional to the input signal. The filter filters the block wave signal to a power output signal. The modulation frequency may be derived from an oscillator, or the amplifier may be self-oscillating. Further, the power amplifier comprises a control circuit. For achieving a high quality power output signal, the control circuit has to control the amplifier in order to compensate numerous non-ideal features of the components of the class D amplifier. Several prior art options for such control circuits are described, and a cascaded control circuit is proposed in the above U.S. patent. The control circuit has a first feedback loop from the output power signal to the input for controlling the operational frequency behavior of the amplifier. The oscillation is controlled by a second feedback circuit coupled between the block wave signal and an input of the modulation circuit. The cascaded control circuit is complicated and requires both feedback circuits to be designed and tuned accurately.